Visual C++ 6.0 treats std::time_t as a 32-bit integer, whereas in Visual Studio 2005 this type is a 64-bit integer. This change might necessitate code updates if your app assumes a 32-bit time_t e.g., hard-coded buffer sizes and format flags (further information on migrating to 64-bit code is available here). Another ABI change affects the representation of the wchar_t type. Up until now, wchar_t has been a typedef synonymous with unsigned short. However, Visual Studio 2005 treats wchar_t as a built-in type. This change might affect the overload resolution of functions:

int _wtolowers(unsigned short *ws);
wchar_t ws[10] = L"Test";
_wtolowers(ws);

All and Sundry
The changes described here affect not just Visual C++ 6.0 code ported to Visual Studio 2005. Rather, they apply to legacy C++ code ported to an ISO compliant C++ compiler in general. The fear from these changes has led many project managers to defer upgrades at all costs. This policy isn't justified, though. Almost without exception, these changes are only for the best.